Europhysics News

Highlights

Swarm-based simulation strategy proves significantly shorter (Vol. 49, No. 1)

Water droplets we used as a test case in this paper

New method creates time-efficient way of computing models of complex systems reaching equilibrium.

When the maths cannot be done by hand, physicists modelling complex systems, like the dynamics of biological molecules in the body, need to use computer simulations. Such complicated systems require a period of time before being measured, as they settle into a balanced state. The question is: how long do computer simulations need to run to be accurate? Speeding up processing time to elucidate highly complex study systems has been a common challenge. And it cannot be done by running parallel computations. That’s because the results from the previous time lapse matters for computing the next time lapse. Now, the authors have developed a practical partial solution to the problem of saving time when using computer simulations that require bringing a complex system into a steady state of equilibrium and measuring its equilibrium properties. These findings have been recently published. One solution is to run multiple copies of the same simulation. In this study, the authors examine an ensemble of 1,000 runs—dubbed a swarm. This approach reduces the overall time required to get the answer to estimating the value of the system at equilibrium.

S. M.A. Malek, R. K. Bowles, I. Saika-Voivod, F. Sciortino, and P. H. Poole, Swarm relaxation: Equilibrating a large ensemble of computer simulations, Eur. Phys. J. E, 40, 98 (2017)
[Abstract]

Swirls study to optimise contacts between fluids (Vol. 43 No. 3)

A representation of the two currents occurring between the concentric cylinders.

A fluid dynamics model of the interface between two swirling fluids gives clues to how to optimise homogeneous feeding of cells in suspension from a liquid nutriments supply in a bioreactor. Studying mixing between two incompatible fluids shows that it is possible to control the undercurrents of one circulating fluid to optimise its exposure to the other. This is the work presented in this article.

The authors compare quantitative experimental observations of a viscous fluid, similar to honey, with numerical simulations. They focus on a fluid, partially filling the space between two concentric cylinders with the inner one rotating, a system previously used to study roll coating and papermaking processes.

They observe the presence of several flow eddies, stemming from fluid flowing past the inner cylinder, causing it to swirl, and the appearance of reverse currents including one orbiting around the rotating cylinder and a second underneath. They make the second eddy disappear by increasing the fluid filling or its velocity.

Instead of using a highly viscous oil combined with air as a top fluid, this model could be applied to a suspension of bioreactor cells typically used to produce biotech medicines, combined with a light oil-containing nutriments as a top fluid. Ultimately, it could help identify the right parameters and adequate mixing time scales to ensure that nutriments feed all the cells homogeneously with no segregation.

Free surface flow between two horizontal concentric cylinders
J. Peixinho, M. Mirbod and J.F. Morris, Eur. Phys. J. E 35, 19 (2012)
[Abstract]

Switch and template pattern formation as in the Drosophila eye (Vol. 42, No. 1)

Spatio-temporal portrait of a moving front in a simple model of switch and template pattern formation. The front moves with a constant speed, leaving behind a highly non-linear, periodic pattern. (For clarity, the spatial patterns at different time points are plotted in different colours.)

Biology provides the physicist with a stunning variety of patterns to explore, and several fundamental ideas in the physics of pattern formation, such as Turing instabilities and the clock-and-wave-front mechanism, are rooted in studies of biological systems. It remains unclear, however, whether these classic concepts can explain the emergence of patterns in most biological systems, or whether new and different mechanisms remain to be discovered.

Here, the authors, at the University of Michigan, Ann Arbor, examined a spatially discrete, three variable reaction-diffusion model inspired by the interactions that create a periodic pattern of gene expression in the Drosophila eye imaginal disc. This model is capable of creating a regular pattern behind a moving front, as observed in eye discs, through a novel "switch and template" mechanism. In order to better understand this mechanism, the authors performed a detailed study of the model's behaviour in one dimension, using a combination of analytic methods and numerical searches of parameter space. Using this approach, the authors find that patterns are created robustly, provided that there is an appropriate separation of time scales and that self-activation is sufficiently strong. Moreover, the paper presents explicit expressions in this limit for the front speed and the pattern wavelength. Moving fronts in pattern-forming systems near an initial linear instability generically select a unique pattern, but the P&L model operates in a strongly non-linear regime where the final pattern depends on the initial conditions as well as on parameter values. This study highlights the important role that cellularisation and cell-autonomous feedback can play in biological pattern formation.

Switch and template pattern formation in a discrete reaction-diffusion system inspired by the Drosophila eye
M.W. Pennington and D.K. Lubensky, Eur. Phys. J. E 33, 2010, 129-148
[Abstract] | [PDF]

Tailoring AlGaInAs quantum dot solar cells (Vol. 43 No. 2)

External quantum efficiency of various solar cell devices: 1) Bulk reference; 2) QD-solar cell without delta-doping; 3) QD-solar cell with delta-doping; 4) Increased number of QD layers; 5) QD-solar cell with delta-doping but reversed QD-arrangement. Inset: Atomic force microscopy image of uncapped AlGaInAs QDs with a remarkably high areal density of 5x10¹⁰ cm^-2.

One of the most important challenges in seeking for competitive energy resources is to increase the efficiency of solar cells. The intermediate band solar cell promises highest efficiencies in a single gap device, since it allows for the absorption of below-bandgap photons via a subband located within the host material's bandgap. In order to generate an isolated subband, electronically coupled stacks of quantum dots (QDs) have been proposed. The built-in potential in the intrinsic region of the solar cell however unavoidably lifts the approximate degeneracy between the QD electron eigenenergies. Hence, the wave functions in adjacent layers would be electronically decoupled, resulting in carrier localization.

To avoid this effect and facilitate the formation of a QD-subband, the present work focuses on the integration of tailored quaternary AlGaInAs QDs. it deals with three major goals in quantum dot solar cell research: Increasing QD absorption by the fabrication of high areal density QDs, engineering the QD's bandgap to tailor their absorption range, and preserving the possibility to maintain an intermediate band despite the built-in potential being present in solar cell p-n-junctions. By a suitable choice of QD composition, shape and barrier thickness they show by simulations routes toward QD-IBSC designs, whereas they experimentally demonstrate a widened spectral absorption range.

AlGaInAs quantum dot solar cells: tailoring quantum dots for intermediate band formation
C. Schneider, S. Kremling, N. V. Tarakina, T. Braun, M. Adams, M. Lermer, S. Reitzenstein, L. Worschech, M. Kamp, S. Höfling and A. Forchel, Semicond. Sci. Technol. 27 032002 (2012)
[Abstract]

Taking advantage of graphene defects (Vol. 45 No.5-6)

The scattering potential in real space calculated based on the Fourier image.
Credit: S. V. Koniakhin

A new theoretical model of the effect of triangular defects in graphene provides numerical estimates of the resulting current rectification with potential applications in security screening.

Electronic transport in graphene contributes to its characteristics. Now, the author is proposing a new theoretical approach to describe graphene with defects—in the form of artificial triangular holes—resulting in the rectification of the electric current within the material. Specifically, the study provides an analytical and numerical theory of the so-called ratchet effect —which results in a direct current under the action of an oscillating electric field, due to the skew scattering of electronic carriers by coherently oriented defects in the material. Such theoretical studies of graphene with triangular defects could be used in the detection of terahertz radiation, which has applications in security screening detectors. These are based on the photogalvanic effect, which is the appearance of electric current as result of irradiation of a device or sample material by light.

S. V. Koniakhin, “Ratchet effect in graphene with trigonal clusters”, Eur. Phys. J. B, 87, 216 (2014)
[Abstract]

Temperature gradients influencing the hysteresis of ferromagnetic nanostructures (Vol. 49, No. 1)

(a) Nanowire device used for magneto-optical Kerr effect (MOKE) measurements. (b) Normalized coercive fields HC/HC,ΔT=0K for Co₃₉Ni₆₁ and Fe₁₅Ni₈₅ nanowires as a function of the temperature gradient ΔT. Increasing HC with increasing ΔT for Co₃₉Ni₆₁ nanowires contradict the basic concept of heat assisted magnetization reversal (HAMR).

For future data storage technology, in which downscaling of magnetic bit unit sizes is crucial, heat-assisted magnetic recording (HAMR) is one key technology to ensure the writability for magnetic bits. It relies on a laser heating pulse to lower the coercive field HC of the magnetic bit unit. Here, we investigated the temperature- and temperature gradient-dependent switching behaviour by HC measurements of individual, single-domain CoNi and FeNi alloy nanowires via measurements of the magneto-optical Kerr effect. While the switching field generally decreased under isothermal conditions at elevated temperatures, temperature gradients (ΔT) along the nanowires led to an increased switching field up to 15 % for ΔT = 300 K in Co₃₉Ni₆₁ nanowires. We attribute this enhancement to a stress-induced contribution of the magneto-elastic anisotropy that counteracts the thermally assisted magnetization reversal process. Our results demonstrate that a careful distinction between locally elevated temperatures and temperature gradients has to be made in future HAMR devices.

A.-K. Michel and 12 co-authors, Temperature gradient-induced magnetization reversal of single ferromagnetic nanowires, J. Phys. D: Appl. Phys. 50, 494007 (2017)
[Abstract]

Temperature-driven ballistic magnon transport (Vol. 49 No.4)

Application of a temperature gradient to a magnetic medium leads to the generation of a spin current referred to as the longitudinal spin Seebeck effect (LSSE). In a magnetic insulator such a current is created by a flux of thermal magnons. Using spin-dependent electron scattering processes in the adjacent normal metal this current can be converted to an electric voltage. The voltage evolution is determined by the development of the temperature gradient ∇T(x,t) and by the characteristics of the magnon’s motion.
By analysis of the time-dependent LSSE voltages in platinum-coated Yttrium Iron Garnet (YIG) ferrimagnetic films, the authors assumed that thermally-driven magnons with energies above 20 K move through the YIG layer ballistically due to their almost linear quasi-acoustic dispersion law. Consequently, the interaction processes within the ‘acoustic’ magnon mode do not change the magnon propagation velocity, while the number of magnons decays exponentially within an effective propagation length of 425nm. This length was found to be mostly independent on film thickness that proves the ballistic magnon transport scenario.

T. B. Noack and eleven co-authors, Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films, J. Phys. D: Appl. Phys. 51, 234003 (2018)
[Abstract]

The 1950s: the decade in which gravity physics became experimental (Vol. 48 No. 1)

Experimental tests from a hundred years ago that compare the gravitational accelerations of different kinds of material

History shows experiments to be just as key as theory in gravity physics In the 1950s and earlier, the gravity theory of Einstein's general relativity was largely a theoretical science. In a new paper published recently, the author shares a historical account of how the experimental study of gravity evolved.

This review examines the broad range of new approaches initiated in the late 1950s, following through to the transition of experimental gravity physics to become a normal and accepted part of physical science in the late 1960s. Highlighting the importance of advances in technology in changing the lines of investigation in the field, it also emphasises the need for physical theories to be empirically tested, because experience shows that this can yield surprising results. In this context, the review examines the role of scientists such as the US physicist Robert Dicke in changing the former perspective. At that time, Dicke made the mid-career decision to lead a research group dedicated to the experimental study of gravity, following new research directions inspired by old arguments associated with Ernst Mach and Paul Dirac.

P.J.E. Peebles, Robert Dicke and the naissance of experimental gravity physics, 1957-1967, Eur. Phys. J. H, (2016)
[Abstract]

The dark side of the optical force (Vol. 44 No. 2)

Lithium and Potassium are the only alkali species possessing stable fermionic isotopes, and as such, they have played a key role in the recent development of quantum simulation of strongly correlated systems using cold atoms. These two species also share an excited hyperfine structure hindering efficient laser cooling below the Doppler limit. In this work, we have implemented a laser-cooling scheme based on dark resonances, which allowed us to achieve record high phase space densities for laser-cooled ⁴⁰K atoms. This strategy was initially developed in the early 90’s and relies on the existence of a family of so-called dark states in which the atoms do not interact with light and do not scatter photons. These states alleviate some of the detrimental effects of traditional schemes, such as spontaneous emission or multiple photon-scattering, which respectively limit the final temperature and density of “bright” optical molasses. This scheme is rather general and can be extended to other atomic species, such as Lithium, as demonstrated by preliminary results obtained in our group.

D. Rio Fernandes, F. Sievers, N. Kretzschmar, S. Wu, C. Salomon and F. Chevy, ‘Sub-Doppler laser cooling of fermionic 40K atoms in three-dimensional gray optical molasses’, EPL, 100, 63001 (2012)
[Abstract]

The Dresden Training for the International nanoCar race (Vol. 48 No. 2)

A 3 step driving along the Au(111) track. The black cross indicates the tip position for the inelastic tunneling excitation of the Dresden molecule-vehicle.

To prepare its participation to the first international nano-car race in Toulouse (France) Spring 2017, the Dresden Team exercised on the Toulouse LT-UHV 4-STM reconfigured for the race with 4 independent controllers (one per scanning tunneling microscope (STM)). An Au(111) surface was prepared over a full gold substrate. A 90 nm long race track with two turns was selected on this surface following the rules (www.cemes.fr/Molecule-car-Race). The Dresden windmill molecule-vehicles were deposited in ultrahigh vacuum conditions, imaged, and manipulated by any one of the 4 tips on race track reaching a 5 nm per hour driving speed,-including the STM image recording after each driving bias voltage pulse. Strategies for a safe and fast driving were established by the Dresden team along the Au(111) surface fcc rafter together with the possibility to repair crashed molecules, for example during the negotiation of a turn. The teams registered for the nano-car race will benefit from this atomic-scale, single-molecule-vehicle driving experience to improve their own driving strategy.

F. Eisenhut, C. Durand, F. Moresco, J.-P. Launay and C. Joachim, Training for the 1^st international nano-car race: the Dresden molecule-vehicle, Eur. Phys. J. Appl. Phys. 76, 10001 (2016).
[Abstract]

The effect of spatiality on multiplex networks (Vol. 47 No. 5-6)

The multiplex structure arising from beginning with nodes on a lattice and connecting them through two layers of links (gray and black) with the length of each link following an exponential distribution

When a node can only form a link to its nearest neighbour, the topology is entirely determined by the spatial locations of the nodes. But when near and far links can form, the influence of the spatial embedding of the topology is much less. In this paper, we use this to modulate the strength of spatial effects on network topology. This allows us to consider the question: Does increasing the allowed geometric length of links in a network improve its robustness? In single-layer networks, the answer is generally that it does. However, in multiplex networks, we find that increasing the link lengths actually makes the network vulnerable to more severe cascade behaviours. This is because in multiplex networks, longer links allow for a discontinuous percolation transition which is characterized by a nucleation process. Our model and results demonstrate the surprising effects of spatial embedding and provide a simple new framework for assessing spatial networks of one or more layers.

M. M. Danziger, L. M. Shekhtman, Y. Berezin and S. Havlin, The effect of spatiality on multiplex networks, EPL 115, 36002 (2016)
[Abstract]

The game of go as a complex network (Vol. 43 No. 4)

Moduli squared of right eigenvectors of the 7 largest eigenvalues of the Google matrix for the first 100 most frequent moves, showing that each eigenvector is localised on specific moves.

The study of complex networks attracts more and more interest, fuelled in particular by the development of communication and information. It turns out that such networks can also modelize many important aspects of the physical world or of social interactions. However, they have never been used in the study of games.

Games have been played for millennia, and besides their intrinsic interest, they represent a privileged approach to the working of human decision-making. They can be very difficult to modelize or simulate: only recently were computers able to beat chess champions. The old Asian game of go is even less tractable, as no computer program has been able to beat a very good player.

The paper presents the first study of the game of go from a complex network perspective. It constructs a directed network, which reflects the statistics of tactical moves. Study of this network for datasets of professional and amateur games shows that the move distribution follows Zipf's law, an empirical law first observed in word frequencies. Differences between professional and amateur games can be seen, e.g. in the distribution of distances between moves. The constructed network is scale-free, with statistical peculiarities, such as the symmetry between ingoing and outgoing links distributions. The study of eigenvalues and eigenvectors of the matrices used by ranking algorithms singles out certain strategic situations (see figure), and vary between amateur and different professional tournaments. These results should pave the way to a better modelization of board games and other types of human strategic scheming.

The game of go as a complex network
B. Georgeot and O. Giraud, EPL, 97, 68002 (2012)
[Abstract]

The importance of rheology in tissue development (Vol. 46 No. 4)

Our understanding of biomechanics increasingly improves through the use of physics models. There are some intriguing biological questions regarding the interplay between the behaviour of cells and the mechanics at the level of tissues. For example, how does a collective behaviour, not apparent at the cell scale, emerge at the tissue level? Or how can the mechanical state of a tissue affect the cell division rate or the orientation of cells undergoing division?

The authors think that the interplay between genes and mechanics is key to understanding how the adult shape emerges from a developing tissue.

They construct rheological diagrams based on insights concerning the mechanics of the biological tissue. One of the main insights is a distinction between intra-cellular and inter-cellular mechanism. The local rheological equations obtained allow to generate a complete spatial model expressed as a set of partial differential equations. This procedure is conducted not only in the case of small elastic deformations, but also in the relevant, less discussed, case of large elastic deformations. The authors provide a functional and versatile toolbox for tissue modelling and propose a framework for a tensorial treatment of heterogeneous tissues. Although the simplest applications concern in vitro experiments, the same approach may be used for many other living tissues including animal tissues during development, wound healing, or carcinogenesis.

S. Tlili, C. Gay, F. Graner, Ph. Marcq, F. Molino and P. Saramito, Colloquium: Mechanical formalisms for tissue dynamics, Eur. Phys. J. E, 38, 33 (2015)
[Abstract]

The neutron-rich superheavy element 116 confirmed (Vol. 43 No. 4)

Cross-sections and cross-section limits of the reaction ⁴⁸Ca + ²⁴⁸Cm → ²⁹⁶116* measured elsewhere and in this work. The data for synthesis of ²⁹³116 (3n channel, triangles) and ²⁹²116 (4n channel, squares) are shown. The experimental data are compared with results of theoretical calculations.

The synthesis of a superheavy element with the proton number Z=116 has been studied at the velocity filter SHIP of GSI in Darmstadt using a ⁴⁸Ca beam on radioactive ²⁴⁸Cm targets. At excitation energies of the compound nuclei of 40.9 MeV, four decay chains were measured, which were assigned to the isotope ²⁹²116 produced in 4n channel, and one chain, which was assigned to ²⁹³116 produced in 3n channel. All chains are terminated by spontaneous fission decays of either ²⁷⁷Hs or ²⁸⁴Cn isotopes on the shoreline of the neutron-rich superheavy island.

Measured cross-sections of 3.4 pb and 0.9 pb, respectively, and decay data of the chains confirm previous data at the Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna. As a new result, one alpha-decay chain was measured, which terminates after four alpha decays by spontaneous fission. The alpha energies of the second to fourth decay are considerably higher than those measured for the alpha decays of ²⁸⁹114, ²⁸⁵Cn, and ²⁸¹Ds and the spontaneous fission half-life is significantly longer than that of ²⁷⁷Hs measured in previous experiments.

Possible assignments and role of isomeric states are discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from ²⁹³116.

The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP. For this purpose, related very detailed experimental study of sources of background fission events was also carried out and published in a related article. Here it was shown that such events occur mainly in connection with transfer reactions leading to target-like residues having half-lives similar to the ones of superheavy isotopes from respective fusion reactions.

The reaction ⁴⁸Ca + ²⁴⁸Cm → ²⁹⁶116* studied at the GSI-SHIP
S. Hofmann et al. (38 co-authors), Eur. Phys. J. A, 48, 62 (2012)
[Abstract]

The source of “background” fission events in experiments on superheavy elements
S. Heinz et al. (9 co-authors), Eur. Phys. J. A, 48, 32 (2012)
[Abstract]

The new frontier in plasma medicine (Vol. 46 No. 3)

Data on the transport of electrical charges in water vapour provide the key ingredients to new plasma models applicable to medicine.

Applications of plasmas in medicine are a new frontier in therapeutic treatment. For example, they can help in stimulating tissue regeneration in the contexts of wound healing and dermatology. Before these and further applications can be developed, it is essential to understand the processes at work in plasmas—a unique kind of gas-like state of matter containing charged particles. Now a study published by the authors provides previously unavailable data on oxygen ion transport and the likelihood of such ions interacting with water molecules. These could contribute to new models of plasmas in liquids which account for how discharges are created in water vapour.

V. Stojanović, Z. Raspopović, D. Marić and Z. Lj. Petrović, Cross sections and transport of O- in H2O vapour at low pressures, Eur. Phys. J. D 69, 63 (2015)
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